Infant's imitation has been proposed as a significant contributor to their language and cognitive development, and intervention programs for children with disabilities have encouraged such children to imitate. Because information on imitation by normal infants and mothers is prerequisite to developing the most effective interventions for children with disabilities, the long-term goals of the research are (A) to identify the causes of normal infants' and mothers' individual differences in imitation, and (B) to determine the consequences of these differences in imitation for infants' development of language and cognition. This project will investigate the origins and outcomes of individual differences in spontaneous verbal and action imitation in a longitudinal sample of mothers and infants observed at 10, 13, 17, and 21 months. Three studies of origins will examine whether mothers' and infants' imitation rates are predicted by general characteristics of mothers' interactive behaviors, by pragmatic and attentional aspects of mothers' speech, or by opportunities for behavior matching provided by partners. Two studies of outcomes will ascertain in infants' initiation of mothers' novel, but not familiar, words predicts their vocabulary development and if mothers' and infants' selective imitation of their partners' most cognitively advanced actions predicts infants' level of non-imitative symbolic play. Findings from these proposed studies may lead to development of more effective interventions to facilitate language and cognitive development in children with disabilities.